It is important that manhole covers be locked in position for a number of different reasons. For example, without being locked in position manhole covers can become dislodged simply by vibrations in the road surface to which they are fitted. Also, they are subject to vandalism and theft.
Conventional manhole covers include vent holes which allow unauthorized prying of the covers from the manhole frame. In some instances, young children after removing a manhole cover have climbed down into the sewer system below where they have been caught up in fast flowing water and drowned. Also, the opening of the manhole covers allow unauthorized persons to have undesirable access to the sewer system.
There are presently in use several different manhole locking devices. One design is a J-bolt which goes through the existing manhole hole and bolts the frame to the cover. Another design is in the form of a water tight cover which again bolts the manhole to the frame.
In both of the arrangements described immediately above, the manhole cover is sealed and therefore does not allow the upward escape of gases past the manhole cover. This can and has resulted in explosions within the sewer system beneath the manhole cover.
In addition to being unsafe, the presently used techniques for locking a manhole cover in position are very labour intensive with respect to removing the covers for maintenance and/or inspections.
Other locking systems for manhole covers have been proposed in prior art patents. Such proposals are shown for example in U.S. Pat. Nos. 3,921,494, 3,772,828, 4,577,478, 4,964,755, 4,902,165, and 2,363,567.
The problem with the structures described in the patents noted immediately above, is that they are not self locking and therefore are subject to human error, i.e. even an authorized person opening the cover could well inadvertently forget to lock the cover.
The cover plate in U.S. Pat. No. 2,363,567 does show gravity operation of a lock bar on a manhole cover for moving the lock bar to a locking position. However admitted in the patent, gravity is not sufficient to ensure proper locking of the bars and therefore a locking key is additionally provided to move the bars to a locking position. The gravity operation of the bars requires that they not only move in and out but also up and down making them relatively loose fitting and complicated in their operation.